CS 380: ARTIFICIAL INTELLIGENCE

ADVERSARIAL SEARCH

Santiago Ontañónso367@drexel.edu

Recall: Problem Solving• Idea: represent the problem we want to solve as:

• State space• Actions• Goal check• Cost function

• If this is possible, we can use search methods to solve any problem:• Systematic Search:

• BFS, DFS, ID, Greedy Search, A*

• Local Search: • Hill Climbing, Simulated Annealing, Local Beam Search (e.g., GAs)

Games vs. search problems

“Unpredictable” opponent ⇒ solution is a strategyspecifying a move for every possible opponent reply

Time limits ⇒ unlikely to find goal, must approximate

Plan of attack:

• Computer considers possible lines of play (Babbage, 1846)

• Algorithm for perfect play (Zermelo, 1912; Von Neumann, 1944)

• Finite horizon, approximate evaluation (Zuse, 1945; Wiener, 1948;Shannon, 1950)

• First chess program (Turing, 1951)

• Machine learning to improve evaluation accuracy (Samuel, 1952–57)

• Pruning to allow deeper search (McCarthy, 1956)

Chapter 6 3

Types of games

deterministic chance

perfect information

imperfect information

chess, checkers,go, othello

backgammonmonopoly

bridge, poker, scrabblenuclear war

battleships,blind tictactoe

Chapter 6 4

Game tree (2-player, deterministic, turns)

XXXX

XX

X

XX

MAX (X)

MIN (O)

X X

O

OOX O

OO O

O OO

MAX (X)

X OX OX O XX X

XX

X X

MIN (O)

X O X X O X X O X

. . . . . . . . . . . .

. . .

. . .

. . .TERMINAL

XX−1 0 +1Utility

Chapter 6 5

Minimax

Perfect play for deterministic, perfect-information games

Idea: choose move to position with highest minimax value= best achievable payoff against best play

E.g., 2-ply game:MAX

3 12 8 642 14 5 2

MIN

3

A 1 A 3A 2

A 13A 12A 11 A 21 A 23A 22 A 33A 32A 31

3 2 2

Chapter 6 6

Minimax algorithm

function Minimax-Decision(state) returns an actioninputs: state, current state in game

return the a in Actions(state) maximizing Min-Value(Result(a, state))

function Max-Value(state) returns a utility valueif Terminal-Test(state) then return Utility(state)v←−∞for a, s in Successors(state) do v←Max(v, Min-Value(s))return v

function Min-Value(state) returns a utility valueif Terminal-Test(state) then return Utility(state)v←∞for a, s in Successors(state) do v←Min(v, Max-Value(s))return v

Chapter 6 7

Properties of minimax

Complete??

Chapter 6 8

Properties of minimax

Complete?? Only if tree is finite (chess has specific rules for this).NB a finite strategy can exist even in an infinite tree!

Optimal??

Chapter 6 9

Properties of minimax

Complete?? Yes, if tree is finite (chess has specific rules for this)

Optimal?? Yes, against an optimal opponent. Otherwise??

Time complexity??

Chapter 6 10

Properties of minimax

Complete?? Yes, if tree is finite (chess has specific rules for this)

Optimal?? Yes, against an optimal opponent. Otherwise??

Time complexity?? O(bm)

Space complexity??

Chapter 6 11

Properties of minimax

Complete?? Yes, if tree is finite (chess has specific rules for this)

Optimal?? Yes, against an optimal opponent. Otherwise??

Time complexity?? O(bm)

Space complexity?? O(bm) (depth-first exploration)

For chess, b ≈ 35, m ≈ 100 for “reasonable” games⇒ exact solution completely infeasible

But do we need to explore every path?

Chapter 6 12

Alpha-Beta Pruning• Not all the nodes in the search tree are relevant for

deciding the next move

5 2 4 1 3 4 2 6 1

Alpha-Beta Pruning• Not all the nodes in the search tree are relevant for

deciding the next move

2

2 1 1

5 2 4 1 3 4 2 6 1

Alpha-Beta Pruning• Not all the nodes in the search tree are relevant for

deciding the next move

2

2 1 1

5 2 4 1 3 4 2 6 1

What would happen is this value was higher?

What would happen if this value was lower?

Alpha-Beta Pruning• Not all the nodes in the search tree are relevant for

deciding the next move

2

2 1 1

5 2 4 1 3 4 2 6 1

What would happen is this value was higher?

What would happen if this value was lower?

NOTHING!

α–β pruning example

MAX

3 12 8

MIN 3

3

Chapter 6 13

α–β pruning example

MAX

3 12 8

MIN 3

2

2

X X

3

Chapter 6 14

α–β pruning example

MAX

3 12 8

MIN 3

2

2

X X14

14

3

Chapter 6 15

α–β pruning example

MAX

3 12 8

MIN 3

2

2

X X14

14

5

5

3

Chapter 6 16

α–β pruning example

MAX

3 12 8

MIN

3

3

2

2

X X14

14

5

5

2

2

3

Chapter 6 17

Why is it called α–β?

..

..

..

MAX

MIN

MAX

MIN V

α is the best value (to max) found so far off the current path

If V is worse than α, max will avoid it ⇒ prune that branch

Define β similarly for min

Chapter 6 18

The α–β algorithm

function Alpha-Beta-Decision(state) returns an actionreturn the a in Actions(state) maximizing Min-Value(Result(a, state))

function Max-Value(state,α,β) returns a utility valueinputs: state, current state in game

α, the value of the best alternative for max along the path to stateβ, the value of the best alternative for min along the path to state

if Terminal-Test(state) then return Utility(state)v←−∞for a, s in Successors(state) do

v←Max(v, Min-Value(s,α,β))if v ≥ β then return vα←Max(α, v)

return v

function Min-Value(state,α,β) returns a utility valuesame as Max-Value but with roles of α,β reversed

Chapter 6 19

Properties of α–β

Pruning does not affect final result

Good move ordering improves effectiveness of pruning

With “perfect ordering,” time complexity = O(bm/2)⇒ doubles solvable depth

A simple example of the value of reasoning about which computations arerelevant (a form of metareasoning)

Unfortunately, 3550 is still impossible!

Chapter 6 20

Resource limits

Standard approach:

• Use Cutoff-Test instead of Terminal-Teste.g., depth limit (perhaps add quiescence search)

• Use Eval instead of Utilityi.e., evaluation function that estimates desirability of position

Suppose we have 100 seconds, explore 104 nodes/second⇒ 106 nodes per move ≈ 358/2

⇒ α–β reaches depth 8 ⇒ pretty good chess program

Chapter 6 21

Evaluation functions

Black to move

White slightly better

White to move

Black winning

For chess, typically linear weighted sum of features

Eval(s) = w1f1(s) + w2f2(s) + . . . + wnfn(s)

e.g., w1 = 9 withf1(s) = (number of white queens) – (number of black queens), etc.

Chapter 6 22

Digression: Exact values don’t matter

MIN

MAX

21

1

42

2

20

1

1 40020

20

Behaviour is preserved under any monotonic transformation of Eval

Only the order matters:payoff in deterministic games acts as an ordinal utility function

Chapter 6 23

Deterministic games in practice

Checkers: Chinook ended 40-year-reign of human world champion MarionTinsley in 1994. Used an endgame database defining perfect play for allpositions involving 8 or fewer pieces on the board, a total of 443,748,401,247positions.

Chess: Deep Blue defeated human world champion Gary Kasparov in a six-game match in 1997. Deep Blue searches 200 million positions per second,uses very sophisticated evaluation, and undisclosed methods for extendingsome lines of search up to 40 ply.

Othello: human champions refuse to compete against computers, who aretoo good.

Go: human champions refuse to compete against computers, who are toobad. In go, b > 300, so most programs use pattern knowledge bases tosuggest plausible moves.

Chapter 6 24

Deterministic games in practice

Checkers: Chinook ended 40-year-reign of human world champion MarionTinsley in 1994. Used an endgame database defining perfect play for allpositions involving 8 or fewer pieces on the board, a total of 443,748,401,247positions.

Chess: Deep Blue defeated human world champion Gary Kasparov in a six-game match in 1997. Deep Blue searches 200 million positions per second,uses very sophisticated evaluation, and undisclosed methods for extendingsome lines of search up to 40 ply.

Othello: human champions refuse to compete against computers, who aretoo good.

Go: human champions refuse to compete against computers, who are toobad. In go, b > 300, so most programs use pattern knowledge bases tosuggest plausible moves.

Chapter 6 24

Nondeterministic games: backgammon

1 2 3 4 5 6 7 8 9 10 11 12

24 23 22 21 20 19 18 17 16 15 14 13

0

25

Chapter 6 25

Nondeterministic games in general

In nondeterministic games, chance introduced by dice, card-shuffling

Simplified example with coin-flipping:

MIN

MAX

2

CHANCE

4 7 4 6 0 5 −2

2 4 0 −2

0.5 0.5 0.5 0.5

3 −1

Chapter 6 26

Algorithm for nondeterministic games

Expectiminimax gives perfect play

Just like Minimax, except we must also handle chance nodes:

. . .if state is a Max node then

return the highest ExpectiMinimax-Value of Successors(state)if state is a Min node then

return the lowest ExpectiMinimax-Value of Successors(state)if state is a chance node then

return average of ExpectiMinimax-Value of Successors(state). . .

Chapter 6 27

Nondeterministic games in practice

Dice rolls increase b: 21 possible rolls with 2 diceBackgammon ≈ 20 legal moves (can be 6,000 with 1-1 roll)

depth 4 = 20 × (21 × 20)3 ≈ 1.2 × 109

As depth increases, probability of reaching a given node shrinks⇒ value of lookahead is diminished

α–β pruning is much less effective

TDGammon uses depth-2 search + very good Eval≈ world-champion level

Chapter 6 28

Digression: Exact values DO matter

DICE

MIN

MAX

2 2 3 3 1 1 4 4

2 3 1 4

.9 .1 .9 .1

2.1 1.3

20 20 30 30 1 1 400 400

20 30 1 400

.9 .1 .9 .1

21 40.9

Behaviour is preserved only by positive linear transformation of Eval

Hence Eval should be proportional to the expected payoff

Chapter 6 29

Games of imperfect information

E.g., card games, where opponent’s initial cards are unknown

Typically we can calculate a probability for each possible deal

Seems just like having one big dice roll at the beginning of the game∗

Idea: compute the minimax value of each action in each deal,then choose the action with highest expected value over all deals∗

Special case: if an action is optimal for all deals, it’s optimal.∗

GIB, current best bridge program, approximates this idea by1) generating 100 deals consistent with bidding information2) picking the action that wins most tricks on average

Chapter 6 30

Proper analysis

* Intuition that the value of an action is the average of its valuesin all actual states is WRONG

With partial observability, value of an action depends on theinformation state or belief state the agent is in

Can generate and search a tree of information states

Leads to rational behaviors such as♦ Acting to obtain information♦ Signalling to one’s partner♦ Acting randomly to minimize information disclosure

Chapter 6 37

Summary

Games are fun to work on! (and dangerous)

They illustrate several important points about AI

♦ perfection is unattainable ⇒ must approximate

♦ good idea to think about what to think about

♦ uncertainty constrains the assignment of values to states

♦ optimal decisions depend on information state, not real state

Games are to AI as grand prix racing is to automobile design

Chapter 6 38